I would bet you get at least 50% more performance with CF 290s than you do with a single 780ti. Watercooling will let you overclock a little higher helping performance, but the big thing is it will help with keeping temperatures down (so they don't throttle) and also noise.
Something you may want to check out if you don't already have a water cooling loop setup and you have room in your case is NZXT's new Kracken GPU bracket. It let's you attach asetek-style CPU coolers to your GPU.